I am trying to understand a bit more about heat pumps and hope some of you can answer some questions

A lot of heat pumps manufacturers are claiming a Co-Efficient of Performance (COP) of 3 to 1 or more

1. So if I understand that correctly that 1kW of electricity will generate 3kW of heat?

2. Does that mean a 14kW heat pump will consume 4.6kW when running at full tilt?

Reading the pages on www.energysavingtrust.org.uk website it would seem that you would need a COP of 3 to 1 that to get a financial saving against gas heating.

3. Final question, is the Co-Efficient of Performance constant e.g. are you going to get a COP of 3 to 1 at 10degC and at -10degC. If it is not constant is there a graph or table that shows what/how the COP might change?

I fitted three ASHPs in my home last year. I chose the Mitsubishi Heavy Industry ones over the Mitsubishi ones (yes, different company would you believe) because they had a claimed COP of just over 5, highest of all the makes and units I looked at.

I also chose to buy 2.0/2.5 kw units as opposed to a full 9kW replacement for my oil fired boiler as it gives me a choice of heating as well as built-in backup and redundancy.

I don't know about the actual COP, but I do know that with all 3 units running 24/7 I get enough background heat to be able to keep the boiler stat low, good air circulation throughout the house, and damp levels which are dropping in a couple of the walls, and my oil bill has dropped by about £800- £1000 this year. So at £500 each (and bearing in mind I had the benefit of fitting them myself for free), payback time will be short.

Looking at my energy meter right now: about 18p/hour for all 3. I had an E7 meter fitted just before installing these, and by using the timers to make full use of E7, you can turn them up a bit higher at night so you don't need the boiler in the morning.

There is some bad news: ASHP efficiency falls off a cliff when the temperature drops to 3-4 degrees. This is because the units have to defrost when the fins on the outside units get clogged. You are then using energy solely to restore the unit to working condition, so your COP probably goes a bit negative for 10 (?) minutes or so.

Overall very happy. To answer your question about temperature, at 10 degrees I would keep them on, at -10 I would turn them off and fire up the second woodburner.

I've never heard of anything giving out more than is put in. 300 percent efficient!

It's not putting out more than is put in.

CoP of 3 means it takes 1kW to MOVE (not MAKE) 3kW of heat - usually from outside to inside.

It makes more sense to forget degrees Centigrade and think of Kelvin.

This morning: van frozen and it's -2C, house seems warm at 19 degrees. Whoa! huge difference! OR...
This morning: van frozen and its +271K, house seems warm at +292K. Now if only we could lift some of that outside heat up to the level of the inside heat. Hmmm, shouldn't be hard, there's less than 10% difference...... Doing the work of moving the heat is where your 1KW goes in the heat pump.

You should really use the seasonal performance factor rather than just COP, the SPF considers the ratio of annual useful heat produced compared to the energy supplied, obviously COP reduces when the external temperature drops, The COP from a ASHP would probably only be only 1:1 this morning at best.

SPF takes into account the de-icing cycles and power used in fans etc.
Part L requires ASHP heating and hot water systems to have a SPF of 2.7 for new and 2.5 for retrofit installations.

With a COP of 3, a heat pump will put 3kw of heat into the heated space for every 1kw of electrical input.
The "extra" heat is extracted from the outside air, soil, or sometimes water.

The claimed COP is often based on optimistic assumptions and is not allways achieved in practice.

If a heat pump is reffered to as "14 kw" this does not make it clear if this is the electrical input, or the claimed heat output.
For domestic use it is PROBABLY the claimed heat output, a domestic unit is most unlikely to have a 14kw input.

The OCPD and cable size should be in accordance with the suppliers instructions, and not sized by any simple ratio from the claimed heat output.

You should really use the seasonal performance factor rather than just COP, the SPF considers the ratio of annual useful heat produced compared to the energy supplied, obviously COP reduces when the external temperature drops, The COP from a ASHP would probably only be only 1:1 this morning at best.

SPF takes into account the de-icing cycles and power used in fans etc.

Part L requires ASHP heating and hot water systems to have a SPF of 2.7 for new and 2.5 for retrofit installations.

Regards

Exactly - perhaps with the additional caveat that SPF is not the basis of determining Ib for the circuit - for that you need the COP at the design day conditon. That could actually be with the heat pump operating in direct resistance heating mode - ie the COP is simply 1.

You need the manufacturers data to determine that design point. Remember there is a difference between power and energy - essentially time. It's the credible power peaks you want for design of the circuit - its the energy values you want to determine any diversity allowable.

Thanks for the very useful replies, as ever there are more questions..

So the COP is just a snapshot and is variable depending on ambient temperature and inlet and outlet temperatures, whereas the SPF is over a whole season (year)

1. SPF - higher is better? - does a SPF of 2.5 mean that the pump will transfer 2.5 more heat then electricity consumed?

2. The manufacturers' data (Ecodan PUHZ-W140VHA) for the pump says...
It has a 14kW capacity
The "running current" is 14.9A with a max of 35.0A and recommend a MCB of 40A
It has a COP of 3.11 at A2/W35
And the input power is 4.52kW at A2/W35
I have not been able to find the SPF.

None of the numbers seem to relate to each other, either from a design of Ib point or a efficiency

I can "crack on" and put in a bit of 6mm and use a 40A OCPD, however if the 14kW heat pump had a COP of 1 would the max current not be 61A, or have the manufactures already applied diversity?

Looking for a solution to the outrages cost of oil heating, (about £110 per month at this time of the year). Thing is this heat pump for example the supply, installation and commissioning cost for an exhaust air heat pump system is £6,250. This would include all the ductwork at 1st fix stage, electrics for the system, connection to the properties services and commissioning. However, for that price and upheavals would be equal to about 10 fills of oil. Other examples of heat pumps costs are not much different.

Thanks for the very useful replies, as ever there are more questions..

So the COP is just a snapshot and is variable depending on ambient temperature and inlet and outlet temperatures, whereas the SPF is over a whole season (year)

1. SPF - higher is better? - does a SPF of 2.5 mean that the pump will transfer 2.5 more heat then electricity consumed?

2. The manufacturers' data (Ecodan PUHZ-W140VHA) for the pump says...

It has a 14kW capacity

The "running current" is 14.9A with a max of 35.0A and recommend a MCB of 40A

It has a COP of 3.11 at A2/W35

And the input power is 4.52kW at A2/W35

I have not been able to find the SPF.

None of the numbers seem to relate to each other, either from a design of Ib point or a efficiency

I can "crack on" and put in a bit of 6mm and use a 40A OCPD, however if the 14kW heat pump had a COP of 1 would the max current not be 61A, or have the manufactures already applied diversity?

I would not "overthink" this.
If the supplier says use a 40 amp OCPD, then I would do just that, and size the cable accordingly.
If the COP declines due to adverse operating conditions, then I would expect either the heat output to decline, or the input current to increase, or both. I would not however expect the input current to exceed the stated maximum of 35 amps, and certainly not to exceed the recomended 40 amp OCPD.

If the supplier says use a 40 amp OCPD, then I would do just that, and size the cable accordingly.

If the COP declines due to adverse operating conditions, then I would expect either the heat output to decline, or the input current to increase, or both. I would not however expect the input current to exceed the stated maximum of 35 amps, and certainly not to exceed the recomended 40 amp OCPD.

That is what I intend to do, however I would like to have a better understanding how the figures are derived and if the claims are justifed - just for my own curosity. Think of it this way, if you were going to put a ASHP in your own home would you not want to know more?

I doubt that the figures actually lie, but they might well be a bit misleading and based on ideal conditions that are seldom achieved.

I suspect that the approx 15 amps running current is for the compressor alone, and that under ideal conditions that this produces enough heat.
I suspect that under less optimum conditions that heating elements are used as well, and that this accounts for the approx 35 amps maximum running current.

I would not put a heat pump into my own home, they can be worthwhile in theory but contain a great deal of high technology that is liable to failure and needs costly repairs.

I would not put a heat pump into my own home, they can be worthwhile in theory but contain a great deal of high technology that is liable to failure and needs costly repairs.

What sort of "high technology" do you have in mind? The ones I've looked at work on the same principle as a fridge (which typically go for decades without any maintenance at all) - and the innards seem a lot less complicated that a typical gas condensing boiler.
- Andy.

Nothing wrong with ASHP if their correctly designed and installed, their suited best to under floor heating systems where lower temps are ok, but larger rads can be installed in theory if underfloor is not possible.

Thanks for the very useful replies, as ever there are more questions..

So the COP is just a snapshot and is variable depending on ambient temperature and inlet and outlet temperatures, whereas the SPF is over a whole season (year)

Essentially yes

1. SPF - higher is better? - does a SPF of 2.5 mean that the pump will transfer 2.5 more heat then electricity consumed?

Higher the better, at SPF 2.5 the heat pump will transfer on average, 2.5 times the electrical input

2. The manufacturers' data (Ecodan PUHZ-W140VHA) for the pump says...

It has a 14kW capacity

OK - that's the sensible heating capacity

The "running current" is 14.9A with a max of 35.0A and recommend a MCB of 40A

OK - the 35A will probably represent the direct resistance element and the pump.

It has a COP of 3.11 at A2/W35

OK - so when the outside air is 2C, the heat pump can deliver 14kW of heating to water at 35C

And the input power is 4.52kW at A2/W35

OK - so at COP 3.11 = 4.25kW x 3.11 = 14kW (approx)

I have not been able to find the SPF.

With Air at 2C and output at 35C I suspect the SPF is actually 3.11 (or reasonably close to it)

None of the numbers seem to relate to each other, either from a design of Ib point or a efficiency

OK - perhaps the above clarifies.

The worst case or design is when the outside air is well below 2C and the output is still required to be 35C - to overcome all the de-icing etc, the system will probably revert to direct resistance heating below about -5C - so that would be a diminished output running direct electric - something like 8kW. So your design figure for Ib is 35A.

I can "crack on" and put in a bit of 6mm and use a 40A OCPD, however if the 14kW heat pump had a COP of 1 would the max current not be 61A, or have the manufactures already applied diversity?

They haven't applied diversity, just a reduced output to match the 35A quoted. How much diversity you apply will be a function of when the heat pump runs - some will be deployed with supplementary heating for extreme conditions (ie very low external temperatures) some won't. Alsothe design flow temperature has an effect - if it's lower than 35C (some UFH circuits for example, or very low temperature large area heated walls) then the COP will improve.

I am trying to understand a bit more about heat pumps and hope some of you can answer some questions

A lot of heat pumps manufacturers are claiming a Co-Efficient of Performance (COP) of 3 to 1 or more

1. So if I understand that correctly that 1kW of electricity will generate 3kW of heat?

2. Does that mean a 14kW heat pump will consume 4.6kW when running at full tilt?

Reading the pages on Link removed website it would seem that you would need a COP of 3 to 1 that to get a financial saving against gas heating.

3. Final question, is the Co-Efficient of Performance constant e.g. are you going to get a COP of 3 to 1 at 10degC and at -10degC. If it is not constant is there a graph or table that shows what/how the COP might change?

Thanks

EDIT: Spelling

Hello Mate,

I might not have permission to answer you here as I am a new bee. I understand a lot about Heat Pumps as I am a Heating Engineerfor over 15 years...

If you want to expand your knowledge about heat pumps of boilders please visit my profile and from there you can understand the whole thing about Heat Pumps as well. Click the link at the right side of my BIO for more details...

Regards,

Ivan

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